1. Field of the Invention
This invention relates to apparatus for reducing noise in moving picture sequence after decoding thereof and, more particularly, an apparatus which permits removal not only of noise at block boundary but also of noise in a block.
2. Description of the Prior Art
As a prior art concerning the reduction of noise in decoded picture, there is one in which the blocking noise is reduced by using LPF (Low Pass Filter) when decoding picture through compression thereof as DCT (Discrete Cosine Transform), as shown in, for instance, Avideh Zakhor, "Iterative Procedure for Reduction of Blocking Effects in Transform Image Coding", IEEE Transactions on Circuits and Systems for Video Technology, vol. 2, No. 1, March 1992, pp. 91-95).
In this prior art system, as shown in FIG. 10, a band limiting process in a band-limitation constraint circuit 31 and a quantization limiting process in a quantization constraint circuit 32 are performed repeatedly k times (k being a positive integer) for blocking noise reduction. Designated at 33 is a process execution times counting circuit. As the band-limitation constraint process, a LPF process of 3.times.3 taps is applied for decoded picture. As the quantization constraint process, on the other hand, the decoded picture is transformed again for each block through DCT into frequency components, the coefficient Ci,j,n (n being the number of times of process execution) is compared with coefficient Ci,j,o obtained in a decoding process after DCT, and a limit is provided lest the difference should exceed the permissible quantization step .DELTA.Q. Subsequently, the coefficient is restored to picture data through IDCT. In the prior art system, when the band-limitation constraint process and quantization constraint process have been repeatedly executed k times, the processing is ended, and picture is output.
In the prior art, blocking noise removal is performed by the filtering process and DCT and IDCT processes repeated 5 to 20 times (i.e., k=5 to 20). This is effective in case where a single frame of picture can be processed for long time as in the still picture. Where it is necessary to process as many as 30 frames per second as in moving picture sequence, however, carrying out such repeated process for each frame gives rise to a problem of extreme load on hardware.
Further, with an increase of the quantization step the permissible quantization step is increased, thus increasing the number of times of the blocking noise reduction filtering to increase the number k of times of repeated operation.
Further, in pictures that have been compression coded by the DCT or the like, not only block-like distortion (i.e., noise at block boundary or blocking noise) but also distortion around edges of characters or the like subject to great brightness changes (i.e., Mosquito noise) is generated. With the prior art system, only the former distortion, i.e., blocking noise, is considered, that is, it is impossible to remove the latter distortion.